Validate base64 input and avoid unwrap panic

[peter-jerry-ye]

base64_decode assumes well-formed input and uses unwrap on get_char, which will panic on invalid indices or malformed strings. It also silently accepts invalid characters.

Suggested fix: validate input length (multiple of 4), handle invalid characters gracefully, and return an error (or Option/Result) instead of panicking.

Refs: lib/json_utils.mbt.